An ideal expected objective of people for a wireless communications system is: A signal is transmitted in a channel in a distortionless manner, and effective information can be obtained at a receive end through efficient, fast, and fully correct decoding. In an actual system, because a real channel environment is relatively complex, some information is usually distorted in a signal transmission procedure. To obtain effective information through full decoding, a transmit end and a receive end need to use a relatively large transmit power and signal-to-noise ratio to transmit a signal, so that information can be correctly decoded at the receive end. However, the transmit power cannot be increased unlimitedly. Therefore, an efficient and correct decoding method needs to be found.
In the prior art, during sequence detection, a Hamming distance is used to select a best path at most time. However, the Hamming distance requires that hard decision processing first be performed on received data to convert the received data into a {0, 1} sequence, and then real data obtained after the decision be compared with ideal data to determine a quantity of same data. A sequence includes only 0 and 1. In an actual system, there is a relatively high probability that two-path data have a same Hamming distance. In addition, the data obtained after the hard decision has a certain error, making it difficult to accurately select the best path, thereby reducing a decoding success rate of the system.